Well, firstly, welcome to the Community modding section. Here, you and others can contribute to the wiki; thus information may not be as heavily detailed and sophisticated as Frictional Games' official details; but as members, we'll do our best to provide precise and detailed steps for you.
This is the Getting Started page, which will contain various links to places which should be of great advantage to you to get introduced into HPL3. Of course, whatever your qualities are, you may be able to get instructions specifically on those since this will be a generic introduction; but once you learn something, ADD IT TO THE WIKI! We need more information, and anything you can provide is beneficial!
Personally, I believe the best way to learn is to see how others do things. In this case, doing some research, understanding how to structure code, exploring levels and determining the relationships between the editors and third party applications will be of great benefits to you. '
Here are some of the things you'll need to get started:
These two mods which were officially released by Frictional Games should also be handy.
And these official pages from Frictional Games within the HPL3 wiki should also be of good use:
In terms of a modification scope, for now, you have two options:
Add-Ons: An addition or modification of files. These are the kinds of modifications which can be broken down into various subcategories - such as texture modifications, GUI mods, script mods, map mods, model mods, sound mods, core modifications… anything really. The aim of an add-on is merely to adjust or, as the name suggests, “add on” to the experience in various ways. Furthermore, it can be just.. different - you could make a playable game of Chess on one of the PCs if you wish!
Stand-Alone Content: These modifications are where your Custom Stories, Full Conversions or even an entirely different game comes in. This is what you're looking for if you're willing to create a campaign for people to play with. You can essentially create whatever you want here as well, but what files and what content you use from SOMA is dependant entirely on you.
In terms of what is included with SOMA, you can use everything and anything as you wish. All assets included with the game are available for use. However, outside the scope of SOMA, there's a few things you should consider when releasing anything. You're allowed to use:
There's not much stopping you from using anything in your mod you're not supposed to should it be a personal project which will not be released in any way whatsoever. However, once something is released, you may be prone to consequences and accusations of using content you shouldn't be. Most websites, including the Frictional Games Forum, Steam Workshop and ModDB, have legal policy agreements which deal with copyright infringement. In most cases, your creations will be removed from being downloaded if you infringe the rights of another individual or company by making use of assets without permission, but it can escalate to termination of accounts and/or legal dilemmas which will occur beyond the scope of this activity.
To avoid such circumstances, it is wise to be aware of the following
–For now, end here. WIP post–